The present invention relates to a process for the treatment of polypropylene with products which are degraded by heat, for forming polypropylene pellets containing such a thermosensitive agent substantially unaffected. The present invention also relates to a process for treating polypropylene to incorporate therein a significant amount of a thermosensitive agent.
More particularly, the present invention relates to a process for the treatment of polypropylene to improve its ability to form fibers or to be formed into fibers.
In addition to applications in the field of fibers, polypropylene treated according to the process of the present invention can be used in other applications such as injection, injection molding or even for the preparation of other compounds and compositions.
It is well known that the ability to form fibers can be achieved with a polypropylene having a narrow molecular weight distribution. Generally, very high molecular weight polypropylenes are produced, which are then degraded chemically or thermally to a certain extent, thus reducing the molecular weight distribution while at the same time reducing the average molecular weight.
Polypropylene is degraded chemically by addition of compounds which decompose into free radicals. Chemical stabilizers added to polypropylene to enhance end-use stability may interfere with free radical generators.
However, it has been found that large free radical generator chemicals, such as the specific types of organic peroxides described in British Patent 1,442,681, are minimally affected by commonly used stabilizers and are thus preferred prodegradents.
The degree to which the polymer can be degraded is limited by the inability of the polymer manufacturer to form pellets from polymers of very low viscosity. Said very low viscosities are necessary in order either to enhance the productivity of extrusion equipment for ultra-fine fibers, or to obtain fibers having specific mechanical characteristics.
Therefore, the polypropylene processor manufacturing films and fibers faces the problem of having to use a polypropylene not optimally suited for these applications. Thus, a need has been demonstrated for a polymer having high viscosity properties for pelletizing purposes, and low viscosity properties for end use processing purposes.
It has already been suggested that the end use processor should add an additional chemical prodegradent to polypropylene pellets to reduce the polymer viscosity to the desired level prior to fiber or film formation. However, there are several disadvantages to this approach:
The peroxide prodegradents are fire/explosion hazards and require special handling procedures and equipment. PA0 To be most effective, the peroxide must be uniformly dispersed within the polymer before it decomposes and reacts, otherwise, a polymer with variable viscosity may result in an heterogeneous weight distribution, which makes impossible any transformation of this polymer into fibers. The polymer producer, having access to specialized apparatus and fine reactor flakes rather than pellets, is in a much better position to achieve the required uniform distribution. PA0 Another disadvantage is that the equipment may be damaged by a variable viscosity polymer. PA0 reduced equipment life; PA0 throughput limitations because of quenching restraints; PA0 excessive energy consumption; and PA0 additive problems including: excessive additive degradation, requiring the inclusion of more additive to the polymer than is required in the final product; limited ranges of useable additives, requiring that more expensive or non-optimal additives be used; and, die capillaries and pipes and the like plugging because of the degradation products. PA0 (i) preparing by extrusion an initial homogeneous blend comprising said polypropylene together with from 5% to 65% of atactic polypropylene; PA0 (ii) introducing into the feed hopper of an extruder the blend as prepared in step (i) simultaneously with of a thermosensitive agent in an amount in the range of between about 0.01 and 20% by weight based on the total weight of polymer; PA0 (iii) extruding the blend and introducing it into an extrusion die and then into a granulator to form polypropylene pellets including a thermosensitive agent; PA0 (iv) recovering the polypropylene pellets thus obtained, which pellets include the substantially intact thermosensitive agent.
Peroxides are more efficient as prodegradents if well dispersed before reacting. Peroxide added to the pellets, rather than within them, acts as a lubricant to extruder feed sections reducing throughput for a given rotation rate.
The processor may also reduce the molecular weight by using very high temperatures to thermally degrade the polypropylene. However, use of very high temperatures leads to:
There has also been proposed a process comprising several successive steps of degradation of a polypropylene, with initial manufacture of a polymer which is easily pelletized, the resulting pellets when heated thereafter being submitted to an additional degradation, thereby producing a polymer of lower viscosity, which may be suitably converted into the form of film or fibers of high quality. However, this type of process is limited by the quantity of prodegradent which can be added. Moreover, it has been noticed that this type of process creates a very bad distribution of the prodegradent since it is introduced into the polymer just before the granulation step to reduce the residence time at high temperature to avoid excessive degradation.